Muscle Mechanics

Are tendons and muscles passive or active elastic ropes individually?

Tendon is passive and muscle is active

What are the 6 different muscle-tendon configurations?

Longitudinal, radiate, fusiform, unipennate, bipennate, and multipennate

What is epimysium?

the outside layer of muscle

What is perimysium?

A layer around the fascicle

What is a fascicle?

a group of five muscle fibers

What is a single muscle fiber cell made of?

myofibril

What is a myofibril made of?

protein actin (Thin filament) and Myosin (Thick filament), which create sarcomere organization

What is the three main types of fibers

Type I, IIa, IIx (slow, fast , very fast)

Slow/fast fiber function

aerobic/anareobic

different fibers have different…

-Rates of force development and contraction
-fuel utilization

What does different training types do to fibers?

cause fibers to hypertrophy, change their energy management

What is a motor unit?

One lower motor neuron and all the muscle fibers it innervates.

What is entheses and myo-tendons?

entheses: bone-tendon junctions by collagen
Myo-tendon: muscle-tendon junctions by protein linkages

How can the brain increase muscle activation?

Recruitment: activate more motor units.

Frequency: stimulate motor units more often
Training the brain activates initial train, and then trains the muscle

What is a muscle twitch?

A brief pulling force produced by one muscle-fibre depolarization

What is tetanus?

Summation of twitch forces caused by repeated depolarization before the muscle fully relaxes

What is unfused tetanus and fused tetanus?

When muscle is fatigued, muscle can recover/ no visible relaxation

Types of EMG and functions

Needle EMG: only one motor unit action potential; needs to be cleaned up
Surface EMG: detects overall activation of all activated units

What is MVA and MVA%, and why is MVC a misnomer?

MVA: maximum voluntary activation, maximum EMG activation you can voluntarily produce
MVA%: EMG during activity/MVA
MVC: Maximum voluntary contraction, relates to force and not electrical activity

What are the three force producers of muscle in Hills model?

actin-myosin activation, viscosity, and parallel elasticity

What is F_A?

-It’s a pulling force that actin and myosin create
-Myosin attaches to sites on the actin, sliding the actin filament alongside it
-causes sacromere to come together

What is the relationship of F_A and Sarcomere Length?

Shortened: too many myosin head too few binding sites, F_A goes down
Lengthened: too few myosin heads many binding sites; F_A goes down
F₀: maximal isometric activation force at resting length

What is F_v?

Pulling or pushing force generated by viscosity of sacromeres

How do the magnitude and direction of F_v relate to velocity?

-The magnitude of F_v is proportional to the magnitude of the velocity of the changing length
-The direction of F_v is opposite to the direction of velocity of the changing length
-isometric: F_v=0

What is F_PE?

-Muscle has tissues around the muscle fibers that can stretch
-produced by the muscle's elastic tissues when the muscle is stretched beyond its resting length
-Non-linear: stiffness increases as muscle gets longer

What are the determinants of muscle force?

muscle activation, normalized length, normalized shortening velocity, ACSA (anatomic cross section)

What is the maximum activation at resting length regarding to ACSA?

90N/cm²

What are normalized length and normalized shortening velocity?

Normalize length=current length/resting length
Normalized shortening velocity=shortening velocity/resting length

What 2 components are affected by normalized length?

F_A and F_PE

What components are affected by normalized shortening velocity?

F_v

Why is the muscle stronger when lengthening?

Because lengthening muscle slows the body down, therefore slowing down is easier than activating force